This invention relates to full width array, multicolor thermal ink jet printers, and more particularly, to a method of extending the life of a printbar in a four printbar, full width array color ink jet printer, by sensing problem nozzles which fail to eject ink droplets and by filling in the missing droplets with ink droplets of a different color ink from one of the other printbars.
It is well known in the reading and/or writing bar industry to assemble pagewidth or full width array raster input scanning (RIS) bars and raster output scanning (ROS) bars from relatively short RIS/ROS subunits placed end-to-end on a structural support bar. Once assembled, the full width array RIS/ROS bars have the requisite length and number of image processing elements to scan or to write an entire line of information with a high image resolution. Image writing bars such as thermal ink jet printbars have a plurality of printhead subunits mounted end-to-end on a structural bar which also serves as a heat sink and may optionally be cooled by passing a liquid therethrough, such as water.
Each thermal ink jet printhead subunit uses thermal energy selectively produced by resistors located in capillary filled ink channels near nozzles or orifices to vaporize momentarily the ink and form vapor bubbles on demand. Each temporary bubble expels an ink droplet and propels it towards a recording medium, such as, paper. The use of an array of printhead subunits is appropriate because full width array printheads cannot be practically fabricated on a single wafer. Full width array printbars may be composed of collinear arrays of printhead subunits or stagged printhead subunits on each side of a structural support bar and offset from one another.
Because full width array printbars composed of collinear arrays of printhead subunits have a number of architectural advantages over staggered offset printbar architectures, this is the configuration used in describing the present invention, though either printbar configuration could be used equally well. One convenient method of fabricating a collinear full width array printbar for an ink jet printer is to butt each printhead subunit against its neighboring printhead subunit. This fabrication method provides very positive positioning of the printhead subunits and minimizes the nozzle gap between adjacent printhead subunits. For such a fabrication method which prevents tolerance stackup, refer to U.S. Pat. No. 5,198,054 to Drake et al.
U.S. Pat. No. 5,192,929 to Drake et al. discloses a means to install full width array printbars into an ink jet printer with the appropriate accuracy to enable alignment of the nozzles of one printbar with the nozzles of other printbars. For multi-color, full width array printers, this is essential because one droplet of one color ejected from one of the printbars must land directly on top of a droplet of a different color ejected from a different printbar.
For a full width array thermal ink jet printbar having 300 nozzles per inch, 2550 total nozzles are required for a recording medium eight and a half inches wide. Statistical variation in resistors or heating elements and their associated ink channels for this quantity of droplet ejecting nozzles clearly predicts that a small number of nozzles will fail to eject droplets long before the other nozzles. As soon as one nozzle fails to eject a droplet, a white line appears in the printed image on the recording medium. This is because the fixed, full width array printbars extend across the recording medium and the recording medium moves therepast at a constant velocity in a direction perpendicular to the linear array of nozzles. For high quality printing, a full width array printbar with one bad or non-droplet ejecting nozzle must be replaced. This invention enables the continued use of printbars with one or more malfunctioning nozzles.